(1) Field of the Invention
The present invention relates to an aperture-limiting element and also an optical head utilizing the same, and more particularly to an aperture-limiting element which permits a single optical system to be compatible with a plurality of different optical recording media and also an optical head utilizing the same.
(2) Description of the Related Art
Compact Disks (CDs) for music reproduction and CD-ROMs for data file read-only optical disk are recently in general use. These disks have a standardized thickness of 1.2 mm. Also, Digital Video Disks (DVDs) which have large capacity and which can reproduce long-time image sequences such as movies have recently been proposed. These DVDs have the same size as the conventional CDs and CD-ROMs, while having large capacity of data recording in reduced-size and high-density pits.
In optical heads for reproducing the DVD, the optical wavelength is made short as compared with that in the prior art, and the numerical aperture (NA) of an objective lens is made large thus reducing a converged beam spot so as to permit reading pits of the reduced size. Specifically, optical heads for the DVDs use a laser with a wavelength of 635 to 650 nm and an objective lens with the NA of 0.6, while those for the conventional CDs and CD-ROMs use a laser with a wavelength of 785 nm and an objective lens with the NA of about 0.45.
Generally, as the NA increases, there is a high degree of increase in the aberration due to the inclination of the optical disk from the optical axis, and this results in enlarging converged beam spots. It is well known in the art that the aberration depends on the thickness of an optical disk substrate, and that its deterioration can be reduced by reducing the thickness. Accordingly, in the DVD, the substrate thickness is set to 0.6 mm. In practice, two substrates each with 0.6 mm thickness are bonded together to provide a total thickness of 1.2 mm, but the light is reflected by the interface between them, i.e., at a depth of 0.6 mm corresponding to the thickness of one of them. Objective lenses for the DVDs are designed in conformity to the substrate thickness of 0.6 mm.
This means that, when an objective lens for the DVDs is used to converge a light beam on an existing CD-ROM having the thickness of 1.2 mm, no sufficiently converged beam spot can be obtained due to spherical aberration, and the CD-ROM cannot be reproduced. That is, an optical head for the DVDs cannot reproduce CD-ROMs. To preclude this inconvenience, various systems, which are compatible with the DVD and the CD-ROM have been proposed.
FIG. 1 shows a prior art example of system introduced in magazines (i.e., Nikkei Mechanical, No. 473, Feb. 5, 1996, p. 16 and Nikkei Electronics, No. 654, Jan. 29, 1996, pp. 15-16).
Referring to FIG. 1, a light beam from a light source 51 is transmitted through a collimating lens 52, a liquid crystal shutter 58 and a beam splitter 53 and converged by a DVD objective lens 56 to be incident on an optical disk 57. The return light beam is reflected by the beam splitter 53, transmitted through a collective lens 61 and a cylindrical lens 62, and received by a photo-detector 63 which detects a reproduced signal, a focus error signal and a tracking error signal.
In CD-ROM reproduction, the liquid crystal shutter 58 provides NA limitation, that is, it limits the aperture of the objective lens to a small value of about 0.35 from 0.6. The liquid crystal shutter 58 is normally fully transparent. In its "on" state, however, the liquid crystal shutter 58 blocks light at a peripheral portion thereof, while leaving at a center portion thereof a circular transparent region. In this way, of the light collimated by the collimating lens, the light at the peripheral portion that causes the generation of the aberration is blocked, and only the light of the center portion is transmitted, so that the aberration is reduced, thus making it possible to reproduce a CD-ROM.
In the DVD reproduction, as shown in FIG. 2A, the liquid crystal shutter 58 is held "off" to provide a fully transparent state with the NA being 0.6.
On the other hand, in the CD-ROM reproduction, as shown in FIG. 2B, the liquid crystal shutter 58 is held "on" to block light so that an NA value becomes about 0.35.
In general, there are two methods of blocking light by utilizing a liquid crystal shutter in the above way, one utilizing whitening of liquid crystal by electric field application, and the other utilizing rotating of polarization in incident light. In the former method, the obtainable contrast is unsatisfactory. For this reason, the latter method will be described in detail with reference to FIGS. 3A and 3B.
Liquid crystals shown in FIGS. 3A and 3B are generally called twisted nematic liquid crystals.
The liquid crystal 66 is sandwiched between a first transparent electrode 65 and a second transparent electrode 67 in the light transmission direction. In the upstream and the downstream of this structure in the light transmission direction, a first polarizer 64 and a second polarizer 68, respectively, are provided such that their polarization directions cross each other.
In the twisted nematic liquid crystal shown in FIG. 3A, when no electric field is set up in the liquid crystal 66 by not applying a voltage between the first and second transparent electrodes 65 and 67, the light transmitted through the first polarizer 64 undergoes optical rotation due to the liquid crystal 66 and is rotated 90.degree.. The light thus rotated is disposed with 90.degree. rotation with respect to the first polarizer 64 and is permitted to pass through the second polarizer 68.
On the other hand, as shown in FIG. 3B, when an electric field is set up by applying a voltage between the first and second transparent electrodes 65 and 67, the polarized light is not rotated by the liquid crystal 66 and is permitted to pass therethrough as it is. Thus, the light transmitted through the liquid crystal 66 is blocked by the second polarizer 68.
The above prior art aperture-limiting element include at least two polarizers and two electrodes with the liquid crystal intervening therebetween. That is, the element requires a large number of components and tends to be increased in scale. This means that an optical head utilizing this element includes a large number of components and tends to be expensive.